All processes described in the prior art for producing sodium dichromate Na2Cr2O7-2 H2O via sodium chromate solution employ a procedure having the same principle:
Chromium spinal or chromite is mixed with sodium carbonate and/or sodium hydroxide and iron oxide (recycled ore residue) and heated at 1000–1100° C. in the presence of oxygen. The sodium chromate produced is leached from the resulting reaction mixture by means of water at a controlled pH. During this procedure, the vanadium present in the chromite also goes into solution as sodium vanadate. Control of the pH is necessary to suppress the dissolution of iron, aluminum, silicon and magnesium. In general, addition of an acid, e.g. a dichromate solution, is necessary to adjust the pH. After leaching with water is complete, the sodium chromate solution produced is converted into a sodium dichromate solution by addition of sulphuric acid or preferably of carbon dioxide under pressure. Solid sodium dichromate is recovered from the solution by evaporation and crystallization. This process is described in BÜCHNER, SCHLIEBS, WINTER, BÜCHEL “lndustrielle Anorganische Chemie”, Weinheim 1984, and in Ullmann's Encyclopedia of Industrial Chemistry, Fifth ed., Vol A 7, Weinheim 1986, p. 67–97.
It has now been found that the vanadium content of the sodium dichromate (about 0.2% of V2O5 in the Na2Cr2O7. 2 H2O) interferes in various applications of the sodium dichromate and its downstream products, so that purification of the sodium chromate fusion solution to remove the vanadium before conversion into sodium dichromate is made necessary.
The removal of vanadium is preferably carried out by addition of calcium oxide to the sodium chromate solution produced by leaching and filtration to remove insoluble material. The solution is in this way brought to a pH of 12–13 (EP-A-0 047 799, EP 0 453 913 B1), resulting in precipitation of a filterable calcium hydroxyvanadate Ca5(OH)(VO4)3. The removal of the calcium which has been introduced in excess is carried out by subsequent precipitation of calcium carbonate from the sodium chromate solution, as described in EP 0 453 913 B1.
A consequence of the precipitation of calcium hydroxyvanadate from a solution having a high concentration of chromate ions is the high contamination of the calcium hydroxyvanadate by coprecipitated calcium chromate and sodium chromate and by entrained calcium oxide. The V2O5 content of the dried “calcium vanadate” precipitate is about 10–20% and is thus significantly below the V2O5 content of pure calcium hydroxyvanadate Ca5(VO4)3OH of 48.7%, but also significantly higher than the V2O5 content of naturally occurring vanadium-containing ore of not more than 2.4% of V2O5 (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Vol. A27, p. 370). The “calcium vanadate” precipitate is thus an attractive starting material for producing vanadium oxide, due to its high vanadium content and high reactivity due to its finely divided nature and low crystallinity. In addition, the disposal of such a material as waste in a landfill is undesirable because of the release of chromate and vanadate to the environment which would occur over time, and the material therefore has to be made inert by treatment with reducing agents such as iron(II)sulphate or sulphur dioxide or the like before it can be disposed of in a landfill.
In the processing of this calcium vanadate precipitate, its valuable constituents, namely vanadium as V in the oxidation state +5, and chromium as Cr in the oxidation state +6, have to be obtained as separate or easily separable species; the vanadium in particular as a readily usable and commercial grade vanadium chemical and the chromium as utilizable, e.g. able to be reintroduced into the chromate production process, i.e. recyclable, species or solution; the readily soluble sodium which is undesirable in the wastewater as a utilizable, recyclable species and the calcium as an insoluble species which can be disposed of in a landfill or used as precursor of a reusable calcium oxide or reusable calcium chromate or calcium dichromate.
Solutions containing sodium ions and chromium in the oxidation state +6 can be reintroduced into the chromate production process only if they do not contain appreciable amounts (e.g. >0.1%) of interfering extraneous material. However, any elements and oxidation states which are not already present to an appreciable extent in the product stream into which this solution is to be introduced are problematic. For this reason, only water H2O, hydroxide OH−or hydroxonium H3O+ions, carbonate, bicarbonate, carbon dioxide, chromate, dichromate, polychromate, chromic acid, sodium and also calcium and vanadate in very minor concentrations are acceptable as constituents of these solutions which are to be reintroduced. Depending on the pH, these solutions can be introduced into the acidification steps of the sodium chromate process (e.g. after or during leaching of the furnace clinker) or the alkalization steps (e.g. before or during vanadate precipitation).
The digestion of the calcium vanadate precipitate by means of sulphuric acid, separation of the precipitated calcium sulphate from the solution in accordance with a treatment of insoluble calcium salts which is frequently practised in industry, and subsequent precipitation of the vanadium as V2O5 from the filtrate by means of sulphuric acid is one way of removing virtually all of the vanadium.
However, a consequence is that the chromium is obtained as a polychromate or chromic acid solution having a high sulphate content. As the many proposals for removing sulphate from sodium chromate and dichromate demonstrate, the amounts of sulphate always introduced as sulphuric acid in earlier processes for producing sodium chromate and sodium dichromate are nowadays totally undesirable for further processing of these products (EP 0 453 913 B1).
Another possible way of treating sparingly soluble salts is digestion of the calcium vanadate precipitate by means of sodium carbonate in aqueous solution and subsequent precipitation of the vanadium as ammonium metavanadate (NH4)4V4O12, also referred to as NH4VO3 for short, by addition of an excess of ammonium salts. Ammonium metavanadate is a versatile intermediate, in particular in the route to the most important vanadium chemical V2O5 (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A27, Weinheim 1996). However, in this case too, the resulting solution containing the chromium as chromate is contaminated with the ions of the precipitation reagent, i.e. with ammonium ions and the associated anions, and can therefore not be reused or recycled without problems. Furthermore, the proportion of the vanadium in the calcium hydroxyvanadate which is dissolved by means of sodium carbonate is less than 50% and therefore completely unsatisfactory unless economically nonviable sodium carbonate excesses are employed.
The routes known from the prior art are therefore not suitable for dissolving vanadium from the calcium vanadate precipitate obtained from the sodium chromate production process in such a way that readily utilizable and commercial grade vanadium chemical can be obtained, and the valuable chromium present also obtained as a usable or recyclable product according to the above-described requirement.